1. Field of the Invention
This invention relates in general to cell relay devices, and more particularly to a method and apparatus for performing switch-over in an intermediate cell switching entity that eliminates packet loss by preventing frame integrity from being broken during the switch-over.
2. Description of Related Art
Cell relay refers to any data communications technology that is based on transmission of small, fixed-length data units called cells. Cells are made up of a header field, containing address information, and an information field, carrying user data. With cell relay, a cell converter takes the constant-bit-rate streams of voice and video devices and the variable-bit-rate streams of data devices and converts them into cells. These cells are then routed across a cell relay network to multiple destinations, based on the address information in the headers. At each destination, other cell converters recreate the bit streams and deliver them to the user devices. With cell relay, separate applications can be consolidated onto a single, higher speed network, improving overall price/performance. Furthermore, because a cell relay network is based on switching fixed-length data units, it is possible to build high performance switching fabrics supporting burst rates of hundreds of megabits per second.
Asynchronous Transfer Mode (ATM) is just one example of a cell-based switching and multiplexing technology. ATM is designed to be a general-purpose, connection-oriented transfer mode for a wide range of services. Today, ATM is being used on local area networks (LANs), metropolitan area networks (MANs) and wide area networks (WANs). As a result, ATM is rapidly becoming the premier protocol for many communication and networking applications. With ATM installed on LANs, MANs and WANs, all types of voice, data and video traffic can operate together seamlessly. No other protocol offers this seamless integration of information, making ATM a catalyst for technological advances in telecommunications, multimedia and other markets.
ATM handles both connection-oriented traffic and connectionless traffic through the use of adaptation layers. Typically, ATM virtual connections operate at either a Constant Bit Rate (CBR) or a Variable Bit Rate (VBR). Each ATM cell sent into the network contains addressing information that establishes a virtual connection from origination to destination. All cells are then transferred, in sequence, over this virtual connection. ATM provides either Permanent or Switched Virtual Circuits (PVCs or SVCs). ATM is asynchronous because the transmitted cells need not be periodic as time slots of data as in Synchronous Transfer Mode (STM).
ATM offers the potential to standardize on one network architecture, which defines the multiplexing and switching method. ATM also supports multiple Quality of Service (QoS) classes for differing application requirements on delay and loss performance. Thus, the vision of ATM is that an entire network can be constructed using ATM and ATM Application Layer (AAL) switching and multiplexing principles to support a wide range of all services, such as:
Voice PA1 Packet data (Switched MultiMegabit Data Service (SMDS), Internet Protocol (IP), Frame Relay (FR) PA1 Video PA1 Imaging PA1 Circuit emulation
ATM also provides bandwidth-on-demand through the use of SVCs, and also supports LAN-like access to available bandwidth.
ATM standards define a fixed-size cell with a length of 53 octets (or bytes) comprised of a 5-octet header and a 48-octet payload. With a relatively small cell size, ATM is a compromise between the long frames generated in data communications and the short, repetitive transmissions required for voice communications, video transmission and other isochronous data transmission.
The bits in the cells are transmitted over the transmission path in a continuous stream. Cells are mapped into a physical transmission path, such as the North American Digital Signal Level 1 (DS1), DS3, or SONET; International Telecommunications Union-Telecommunications standardization sector (ITU-T) SDH standards; and various other local fiber and electrical transmission payloads.
All information is switched and multiplexed in an ATM network using these fixed-length cells. The cell header identifies the destination, cell type, and priority. Fields of the cell header include: the Virtual Path Identifier (VPI) and Virtual Circuit Identifier (VCI) which hold local significance only, and identify the destination. The Generic Flow Control (GFC) field allows a multiplexer to control the rate of an ATM terminal. The Payload Type (PT) indicates whether the cell contains user data, signaling data, or maintenance information. The Cell Loss Priority (CLP) bit indicates the relative priority of the cell. Lower priority cells may be discarded before priority cells during congested intervals.
Because of its critical nature, the cell includes a Header Error Check (HEC) which detects and corrects errors in the header. The payload field is passed through the network intact, with no error checking or correction. ATM relies on higher layer protocols to perform error checking and correction on the payload. The fixed cell size simplifies the implementation of ATM switches and multiplexers while providing very high speeds.
When using ATM, longer packets cannot delay shorter packets as in other switched implementations because long packets are chopped up into many cells. This enables ATM to carry Constant Bit Rate (CBR) traffic such as voice and video in conjunction with Variable Bit-Rate (VBR) data traffic, potentially having very long packets within the same network.
Three major concepts in ATM are: the transmission path, the Virtual Path (VP), and, optionally, the Virtual Circuit (VC). These form the basic building blocks of ATM. A physical transmission path contains one or more virtual paths (VPs), while each virtual path contains one or more virtual circuits (VCs). Thus, multiple virtual circuits can be trunked on a single virtual path. Switching can be performed on either a transmission path, virtual path, or virtual circuit (i.e., channel) level.
This capability to switch down to a virtual circuit level is similar to the operation of a Private or Public Branch Exchange (PBX) or telephone switch in the telephone world. In the PBX/switch, each channel within a trunk group (path) can be switched. Devices which perform VC connections are commonly called VC switches because of this analogy with telephone switches. Transmission networks use a cross-connect, which is basically a space division switch, or effectively an electronic patch panel. ATM devices which connect VPs are commonly called VP cross-connects by analogy with the transmission network.
At the ATM layer, users are provided a choice of either a Virtual Path Connection (VPC) or a Virtual Channel Connection (VCC). VPCs are switched based upon the Virtual Path Identifier (VPI) value only. The users of the VPC may assign the VCCs within that VPI transparently since they follow the same route. VCCs are switched upon the combined VPI and Virtual Circuit Identifier (VCI) value.
Both VPIs and VCIs are used to route cells through the network. It should be noted that VPI and VCI values must be unique on a specific transmission path (TP). Thus, each transmission path between two network devices (such as ATM switches) uses VPIs and VCIs independently.
Accordingly, ATM networks will be used quite extensively for data transfer. However, a data network can be built on top of an ATM network so that ATM is used as a technology to interconnect those sites to form an overlay network. Those skilled in the art will recognize that a data network scenario is mentioned herein as an example only. Thus, the present invention is not meant to be limited to data networks.
As suggested above, some data networks are connectionless in nature and run dynamic routing protocols to determine the path through the network. The path that each data flow takes is subject to change for several reasons including the cases where the previous path is not available any more or it has for some reason become non-optimal. The changes in the data network routing will reflect into the underlying ATM network and may generate a need for reconfiguring of connections.
For an ATM switch, a connection modification may refer to the modification of bandwidth resources associated with the connection. However, in this case, the endpoint may remain the same. Nevertheless, a connection modification may also refer to a situation where one of ATM virtual channel link (VCL) termination points, either incoming or outgoing, will be changed in the switch. In this latter case, resources may be the same for both the old and the new connection. In a special case of a connection that is terminated in the switch, the connection topology change is internal to the switch and results in changing the point that handles AAL frames, i.e. AAL connection termination point. Hereinafter, the term switch-over is used to indicate connection modifications where one of the ATM virtual channel link termination points is changed in a switch. Resource allocation will be assumed to be unchanged.
ATM is a scaleable standard that does not specify requirements for transmission rates, framing and physical layers. Rather, ATM switching and ATM networks refer only to the handling of cells. ATM does not dictate the content of information carried in cells. Broadband networks must develop guarantees on bandwidth, delay and jitter to support a wide variety of ATM applications.
As mentioned earlier, ATM cells have a fixed length payload field that is 48 octets. However, data packets that are carried in ATM cells are normally longer than 48 octets and are not integral multiple of 48 octets. Therefore, the ATM Adaptation Layer (AAL) is used to assemble and reassemble those packets into/from cells.
However, if for one reason or the other, some cells are lost or misrouted in the network, the whole frame associated with the lost cell will be worthless. In order to eliminate packet loss resulting from connection changes, the switch-over has to be performed so that frame integrity is not broken. This problem is encountered with all types of cell relay systems.
It can be seen then that there is a need to add additional intelligence to an intermediate switch to prevent packet loss at the switch-over.
It can also be seen that there is a need for a method and apparatus for performing cell switch-over that eliminates packet loss by preventing frame integrity from being broken during the switch-over.